The overall aim of this project is to gain a better understanding of the role of selection in the genetics of natural populations through a study of the dynamic and static behavior of deterministic discrete generation, constant fitness, population genetic models. I will investigate models in which a priori assumptions are not made about the form of the selection coefficients. I will use new methods of analysis based on specifyiing equilibria or changes in samete frequencies, and also on bifurcation theory. I will delimit the relationship among selection, linkage disequilibrium and recombination, both at equilibria and for the dynamical behavior of models, once the details of the mating system have been described. These general results will facilitate the interpretation of experimental data, including human data obtained for loci controlling the HLA system. Employing a similar general approach, I will study the evolution of modifiers. The extension allowing general fitness patterns and nonequilibrium behavior is important here to obtain robust results. I will also extend all of the analysis, as far as is possible, to selection other than viability selection at the diploid stage, extending the applicability of the models.